Polypropylene (PP) is a thermoplastic resin obtained by polymerizing propylene. The PP has excellent characteristics such as a small density, a high strength, and excellent heat resistance and chemical resistance. Therefore, the PP is widely used in various applications such as fiber materials, packaging materials, containers and automobile components.
The PP fiber is usually produced by melt-spinning the PP and by drawing the spun yarn. Patent Document 1 describes a method for producing highly elastic PP, which is characterized in that an undrawn fiber obtained by spinning a crystalline PP having 1 to 5 melt-index value of product is drawn at a temperature close to the softening temperature thereof at a draw ratio close to the maximum draw ratio. This document describes that according to such a method, high elastic PP having 1 to 10 denier fineness in single yarn and an elasticity recovering energy amount of 5.0 kgfm/g or more after repetition of 10 times under load of 80% of the breaking strength can be obtained.
Patent Document 2 describes a method for producing a high strength PP fiber, which is characterized in that highly regular PP having a melt-flow rate of 5 to 15, an isotactic pentad fraction of 95 or more, a fraction insoluble in n-heptane of 98 or more and a Q value (weight average molecular weight/number average molecular weight) of 4 or less as a raw material is melt-spun at a temperature of 270° C. or more, and after that the melt-spun yarn is drawn at a high draw rate of 7 times or more to obtain the breaking strength of 8 g/d or more.
Patent Document 3 describes a method for producing a PP fiber, which is characterized in that an undrawn fiber that is formed of PP having a fraction of isotactic pentad of 96.5% or more at a melt flow rate of 3 to 100 g/10 min and has a ratio of smectic crystals of 30% or more is drawn.
Patent document 4 describes a method for producing a ultrahigh molecular weight PP drawn molded body, which is characterized in that, after a fluidity improver (C) is added to a ultrahigh molecular weight PP composition including 85 to 99.5 parts by weight of ultrahigh molecular weight polypropylene (A) of which limiting viscosity [η] is at least 5 dl/g or more, and 0.5 to 15 parts by weight of polyethylene (B) of which limiting viscosity [η] is at least 2 dl/g or more, and the mixture is melt-mixed, this is extruded from a die and molded, the resulted extruded matter is drawn. This document describes that when ultrahigh molecular weight PP is used, a PP fiber having the tensile strength of at least 0.8 GPa can be obtained.